Hello everyone!
Stella here! I'm newly registered with your community but here I was seen that mostly threads is closed. Why ?

Hey Stella! Great to have you on BI!
Yes, unfortunately all threads eventually close after not being posted on for a certain period of time. (not sure how long.) So, almost all the old threads are closed except ones like this one, that are "stickied" by an admin or mod, to stay open indefinetly.
If you click on New Posts, at the top of the screen, you can see the threads that have been posted on recently.